The present application claims priority of United Kingdom Patent Application Serial No. 0129020.4, filed 4 Dec. 2001, the contents which are hereby incorporated hereby in its entirety.
The present invention relates to the testing of a laminated core of an electrical machine.
The stator cores of generators and other electrical machines are built up of thin steel laminations which are each coated with a layer of electrical insulation. This insulation prevents the alternating magnetic flux, through the core, inducing unwanted eddy currents between the laminations.
The lamination insulation may, however, become damaged during assembly or maintenance of a stator, particularly during removal or replacement of the rotor. The insulation may also degrade during operation, due to, for example, wear of the insulation.
If the insulation becomes damaged, then conducting circuits may be formed between laminations, through which fault currents are induced by the alternating magnetic flux. These damaged regions may become hot in service and are referred to as hot spots. These hot spots may damage the insulation on adjacent sections of the stator and can cause electrical breakdown and failure of the machine.
There have been several techniques developed to identify damaged insulation and so allow repair to reduce the severity of the hot spots. One such test involves generating a large (around 80% of the normal operating value) magnetic ring flux around the stator core. This heats the hot spot allowing infra red equipment to locate the position of the hot spot. This type of test however requires a large amount of power and may not be able to detect deep seated core damage. Additionally, these tests are labour intensive as the excitation windings are large. This in turn has prevented the tests being carried out with the rotor in situ. Other tests have therefore been developed to mitigate these problems. Two examples using different techniques now follow.
A local excitation test is described in U.S. Pat No. 5,990,688. This uses a test head that is a C shaped section of laminated steel held against the stator teeth which locally excites the core. Using this test, only a low excitation flux is required to be generated in the stator. This means that a small amount of power is required to conduct this test. The test is also simple to set up as the test head need only be held against the stator teeth.
However, the test head is relatively large and heavy. This means that the test head cannot traverse the stator core very easily. Additionally, the test may not enable hot spots to be located precisely and so repairing the laminations may be difficult.
A second example is the Electromagnetic Core Imperfection Detector (ElCID) test. This test is described in GB-A-2 044 936 and requires that a low ring flux is generated around the stator core (typically only 4% of the normal operating value). This ring flux is produced by an excitation winding through the bore of the stator core. The ring flux induces fault currents in the stator core that flow through any potential hot spots, but these fault currents are too small to cause any detectable heating. Instead the current flowing through the fault is detected electromagnetically using a special pick-up coil such as, for example, a Chattock Potentiometer. Such coils also detect the magnetic fields produced by the excitation current that is typically much larger than those produced by the fault currents. The fault current tends to be in phase quadrature with the excitation current and so by identifying and measuring the component of the detected current that is in phase quadrature with the excitation current, a fault current can be quantified allowing the severity of the fault to be determined quickly, without the use of large number of personnel. Also it is possible, using ElCID, to locate the position of the fault, facilitating repair as well as providing a permanent record of the fault severity thereby allowing the effectiveness of the repair to be quickly assessed. ElCID testing is now common in the electrical power industries and is widely accepted as a reliable core testing method.
However there are several situations in which it is difficult to obtain reliable results using ElCID testing. These include testing of stators with rotors in situ and the testing of certain areas of hydrogenerator cores.
Rotor in situ testing is reasonably common in hydrogenerators as there is often sufficient access to test the stator bore, without major disassembly of the machine. There can however be difficulties because the current through the excitation winding induces eddy currents in solid steel components such as the rotor and rotor bearings. These eddy currents produce magnetic fields at the tips of the stator teeth that are not in phase with the excitation current and so can affect the ElCID test results such that faults are difficult to identify.
Many hydrogenerators are made up of split cores, where the core comes in two or more sectors which are assembled on site. Inevitably there are small gaps at the joins between the sectors of the laminated core. The ring flux around the core generates very large magnetic fields as it crosses the gaps and so increases the magnetic field at the stator teeth near the gap. The amplitude and phase of these fields vary along the length of the gap and the amplitude of these fields can be much greater than (typically 100 times greater than) those produced by a fault current. Since the phase and amplitude of these magnetic fields are not well defined it becomes very difficult using the standard ElCID test to determine whether there is a genuine fault in the core near the joins. This is a serious drawback as damage is likely to occur at the joins in the core.
It is an object of the present invention to address these problems.